The invention relates to a ship propulsion device.
A traditional ship includes a propulsion propeller and a rudder. Today, there is a trend to use so-called rudder propeller devices of the type described, for example, in Patent Publications DE 26 55 667, SE 412 565, Fl 75128, GB 2 179 312, CA 1,311,657 and U.S. Pat. No. 5,403,216, as the main propulsion means of a ship. A rudder propeller device includes one or several propulsion propellers mounted on a shaft journalled in an underwater housing or pod, which is turnable around a substantially vertical axis. The pod is attached to the lower end of a shaft structure which is turnably journalled in the hull of the ship and is traditionally a straight tubular member. In the following, this shaft structure is called turning shaft. By turning the turning shaft it is possible to direct the pod and thus also the propeller flow in any desired direction. Therefore, a rudder propeller device may also function as the steering device of the ship.
The turning axis of the turning shaft and the pod does not need to be exactly vertical, and it can deviate somewhat from the vertical orientation, for example as described in U.S. Pat. No. 5,403,216.
The steerability of a ship equipped with a rudder propeller device is excellent, but the torque required for turning the pod is high and increases as a function of the propulsion power. The high torque causes problems in particular in slow moving ships with high propeller thrust such as tugs and icebreakers. Problems occur even when the propulsion power per propulsion unit is only some hundreds kilowatts.
Today, the power of a rudder propeller device may be considerable. Rudder propeller devices with a power of more than 20 MW are being designed. In this power class, the torque required for turning the propeller pod reaches high values and thus requires very strong steering machinery, which is a disadvantage.